Novel 5-HT3 receptor antagonists and methods of use

ABSTRACT

The subject invention provides useful and novel 5-HT3 antagonists. The subject invention also provides methods for synthesizing the compounds of invention. The invention also provides methods for the treatment of irritable bowel syndrome and other such conditions.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of co-pending application U.S.Ser. No. 10/348,669, filed Jan. 21, 2003; which claims the benefit ofU.S. Provisional Application Ser. No. 60/350,504, filed Jan. 18, 2002.

BACKGROUND OF INVENTION

[0002] Irritable bowel syndrome (IBS) is one of the most commongastrointestinal disorder thought to result from dysregulation ofintestinal motor, sensory and CNS function. In the United States, theestimated prevalence is 15% to 20%, and 75% of patients are women.Despite its prevalence, IBS is poorly understood. It is one of over 20functional gastrointestinal (GI) disorders that are not explained byidentifiable structural or biochemical abnormalities. IBS ischaracterized by persistent or recurrent symptoms of abdominal pain withdiarrhea and/or constipation. IBS is believed to relate to abnormalitiesin motility and/or afferent sensitivity as mediated by the centralnervous system. Patients with IBS have a diminished quality of life anduse significant health care resources.

[0003] Treatment for patients diagnosed with IBS has includedantidepressant drugs, tranquilizers and laxatives. Pharmacologicalintervention in diarrhea-predominant IBS focuses on the reduction ofbowel motility, spasms and transit times. Peripherally acting opiodligands such as the petidine congeners diphenoxylate and loperamide andthe k-opiod agonist fedotozine slow gastrointestinal transit by theireffects on the circular and longitudinal muscle. While these drugs showsome effects on intestinal motility, their effects on IBS-relatedabdominal pain and intestinal relief is generally insufficient.

[0004] Alosetron, a selective 5-HT3 receptor antagonist closely relatedto ondansetron in terms of chemistry and pharmacology, is the firstcompound of this type to be developed for irritable bowel syndrome.Alosetron and its uses are described in, for example, U.S. Pat. No.6,284,770, which is incorporated herein by reference. A number ofdifferent 5-HT3 receptor antagonists have been disclosed, for examplethose of group A: indisetron, Ro-93777, YM-114, granisetron, talipexole,azasetron, tropisetron, mirtazapine, ramosetron, ondansetron,lerisetron, alosetron, N-3389, zacopride, cilansetron, E-3620,lintopride, KAE-393, itasetron, mosapride and dolasetron.

[0005] In UK Patent No. 2209335 there is disclosed, inter alia, thecompound2,3,4,5-tetrahydro-5-methyl-2-[(5-methyl-1H-imidazol-4-yl)methyl]-1H-pyrido[4,3-b]indol-1-one, now known as alosetron, and pharmaceuticallyacceptable salts, solvates and pharmaceutically acceptable equivalentsthereof, in particular its hydrochloride salt.

[0006] 5-HT3 receptor antagonists are known to be useful in thetreatment of a variety of conditions involving 5-HT3 receptor-mediatedmechanisms, including in particular emesis.

[0007] Ondansetron inhibits emesis by blocking 5-HT3 receptors on vagalafferent nerve terminals in the gastrointestinal mucosa and on terminalson the same vagal nerves in the vomiting system located in the dorsalmedulla of brain stem. Alosetron, in various animal models, can reducethe increase in intestinal fluid secretion and motility triggered byserotonin release. Alosetron increases sensory threshold to balloondistension of the rectum, either by a direct effect on afferent painperception, or via an increase in rectal compliance. In addition, 5-HT3receptor antagonists have been shown to slow colonic transit in man(ondansetron and alosetron). Clinical data for up to 3 months oftreatment indicate that alosetron was orally bioavailable andsignificantly superior to both placebo and the smooth muscle relaxant,mebeverine, in improving perception of visceral pain, spasms anddiarrhea in female diarrhea-predominant IBS.

[0008] Alosetron received FDA approval for the treatment of IBS in womenwith diarrhea in early 2000. It is the first drug to have provenefficacy for IBS. Alosetron (Lotronex™) was launched in the US, itsfirst market and was also launched in Puerto Rico for the treatment ofwomen with irritable bowel syndrome who have diarrhea as the predominantsymptom. However, in November 2000, Glaxo Wellcome withdrew alosetronfrom the US market, prompted by reports of alosetron-associatedischaemic colitis (n=49; characterized by abdominal cramping and pain)and severe constipation (n=21). The FDA also received 3 reports ofdeaths which were associated with alosetron.

[0009] Alosetron has approximately 60% oral bioavailability and ahalf-life of 1.5 hours. Greater variability was seen in thepharmacokinetic profile in all parameters in females compared withmales. Females also had 60% greater drug exposure than males, with meanpeak plasma concentration 45-100% higher. This is attributed to lowerclearance and volume of distribution in female population. Similargender-specific differences have been reported for ondansetron. At least12 metabolites were detected in urine, which were eliminated from plasmawith half-lives of approximately 3 hours. 6-Hydroxy-alosetron, which istwice as potent as alosetron was not detected in plasma, however thelimit of detection was 6-fold higher than the Ki for this metabolite.

[0010] The pattern of fecal and urinary elimination of alosetron and itsmetabolites is suggestive of enterohepatic recirculation of6-OH-alosetron, resulting in “prolonged” low level exposure. Inaddition, 6-OH-alosetron glucuronide and a hydroxymethyl metabolite alsohave potent 5-HT3 receptor binding affinity. The pharmacoynamic effectsof these metabolites are unknown. Clearance was predominantly bymetabolism and renal excretion. Mass balance studies with radiolabeleddrug indicate that the concentration of circulating metabolite is atleast 10 fold greater than that of alosetron, yet, two-thirds of thecirculating radioactivity cannot be attributed to alosetron or itsmetabolites. This is due to slower elimination and smaller Vd of themetabolites.

[0011] Over 1200 patients with IBS received alosetron for at least 12weeks during the Phase II and III clinical trials. Constipation was themost commonly reported adverse event, occurring in 28% of those takingalosetron and in 3% of those on placebo. This side effect appears to bedose dependent and constipation occurred more frequently in femalepatients. This gender difference is perhaps related to the increaseddrug exposure level in the female patients.

[0012] Thus, it would be particularly desirable to find potent andselective 5-HT3 antagonists having comparable pharmacodynamic effect tothat of alosetron, with more predictable metabolism and an improvedsafety profile.

BRIEF SUMMARY

[0013] The subject invention provides useful and novel 5-HT3 antagonistsand methods of use. The subject invention also provides methods forsynthesizing the compounds of the subject invention. In a specificembodiment, the subject invention also provides methods for thetreatment of irritable bowel syndrome.

[0014] Advantageously, the subject invention provides compounds whichare readily metabolized by the physiological metabolic drugdetoxification systems. Specifically, in a preferred embodiment, thetherapeutic compounds of the subject invention contain an ester group,which does not detract from the ability of these compounds to provide atherapeutic benefit, but which makes these compounds more susceptible todegradation by hydrolases, particularly serum and/or cytosolicesterases. The subject invention further provides methods of treatmentcomprising the administration of these compounds to individuals in needof 5-HT3 antagonist treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 shows the structures of three 5-HT3 antagonists—alosetron,ondansetran, and granisetron.

[0016]FIG. 2 shows sites at which alosetron may be modified according tothe subject invention to create analogs (soft chemistry sites) whichhave advantageous pharmacokinetic properties as described herein.

[0017]FIG. 3 shows specific analogs of the subject invention as well asthe primary inactive metabolite after the active compound has beenexposed to hydrolytic enzymes.

[0018]FIG. 4 shows specific analogs of the subject invention as well asthe primary inactive metabolite after the active compound has beenexposed to hydrolytic enzymes.

[0019]FIG. 5 shows specific analogs of the subject invention as well asthe primary inactive metabolite after the active compound has beenexposed to hydrolytic enzymes.

[0020]FIG. 6 shows an example of a synthetic scheme which can be used tosynthesize certain specific analogs of the subject invention.

[0021]FIG. 7 shows an example of a synthetic scheme which can be used tosynthesize certain specific analogs of the subject invention.

[0022]FIG. 8 shows an example of a synthetic scheme which can be used tosynthesize certain specific analogs of the subject invention.

DETAILED DISCLOSURE

[0023] The subject invention provides novel 5-HT3 receptor antagonists.In a preferred embodiment, the 5-HT3 anatgonists of the subjectinvention can be deactivated to a primary inactive metabolite byhydrolytic enzymes.

[0024] Compounds of the present invention can be advantageously used totreat individuals in need of treatment with a 5-HT3 receptor antagonist.In a preferred embodiment, the compounds of the subject invention areused to treat patients suffering from gastrointestinal disorders asexemplified by irritable bowel syndrome. The compounds of the subjectinvention are particularly advantageous due to their predictablepharmacokinetics.

[0025] As used herein, the term “individual(s)” refers to a mammal towhich is administered a compound or composition of the presentinvention. The mammal may be, for example a mouse, rat, pig, horse,rabbit, goat, pig, cow, cat, dog, or human. In a preferred embodiment,the individual is a human.

[0026] Granisetron and ondansetron are potent 5-HT3 antagonists widelyused for treating emesis induced by chemotherapy (FIG. 1). Whileondansetron was the first to demonstrate 5-HT3 antagonism activity inthe colon, thereby reducing the sensation of pain and transit of contentthrough the colon, alosetron was the first compound of this type to bedeveloped for irritable bowel syndrome.

[0027] In a specific embodiment, the present invention provides novelalosetron analogs that are preferentially metabolized by endogenoushydrolytic enzymes. The novel compounds are bioactive molecules havingactivity on the gastrointestinal tract and undergoing deactivation toprimary inactive metabolites by hydrolytic enzymes.

[0028] Sites at which alosetron can be modified according to the subjectinvention are shown in FIG. 2. Specific analogs of the subjectinvention, as well as the primary metabolites are shown in FIGS. 3-5.

[0029] Adverse drug-drug interactions (DDI), elevation of liver functiontest (LFT) values, and QT prolongation leading to torsades de pointes(TDP) are three major reasons why drug candidates fail to obtain FDAapproval. All these causes are, to some extent, metabolism-based. A drugthat has two metabolic pathways, one oxidative and one non-oxidative,built into its structure is highly desirable in the pharmaceuticalindustry. An alternate, non-oxidative metabolic pathway provides thetreated subject with an alternative drug detoxification pathway (anescape route) when one of the oxidative metabolic pathways becomessaturated or non-functional. While a dual metabolic pathway is necessaryin order to provide an escape metabolic route, other features are neededto obtain drugs that are safe regarding DDI, TDP, and LFT elevations.

[0030] In addition to having two metabolic pathways, the drug shouldhave a rapid metabolic clearance (short metabolic half-life) so thatblood levels of unbound drug do not rise to dangerous levels in cases ofDDI at the protein level. Also, if the metabolic half-life of the drugis too long, then the CYP450 system again becomes the main eliminationpathway, thus defeating the original purpose of the design. In order toavoid high peak concentrations and rapidly declining blood levels whenadministered, such a drug should also be administered using a deliverysystem that produces constant and controllable blood levels over time.

[0031] The compounds of this invention have one or more of the followingcharacteristics or properties:

[0032] 1. Compounds of the invention are metabolized both by CYP450 andby a non-oxidative metabolic enzyme or system of enzymes;

[0033] 2. Compounds of the invention have a short (up to four (4) hours)non-oxidative metabolic half-life;

[0034] 3. Oral bioavailability of the compounds is consistent with oraladministration using standard pharmaceutical oral formulations; however,the compounds, and compositions thereof, can also be administered usingany delivery system that produces constant and controllable blood levelsover time;

[0035] 4. Compounds according to the invention contain a hydrolysablebond that can be cleaved non-oxidatively by hydrolytic enzymes;

[0036] 5. Compounds of the invention can be made using standardtechniques of small-scale and large-scale chemical synthesis;

[0037] 6. The primary metabolites of compounds of this invention resultfrom the non-oxidative metabolism of the compounds;

[0038] 7. The primary metabolites, regardless of the solubilityproperties of the parent drug, is, or are, soluble in water atphysiological pH and have, as compared to the parent compound, asignificantly reduced pharmacological activity;

[0039] 8. The primary metabolites, regardless of theelectrophysiological properties of the parent drug, has, or have,negligible inhibitory activity at the IK_(R) (HERG) channel at normaltherapeutic concentration of the parent drug in plasma (e.g., theconcentration of the metabolite must be at least five times higher thanthe normal therapeutic concentration of the parent compound beforeactivity at the IK_(R) channel is observed);

[0040] 9. Compounds of the invention, as well as the metabolitesthereof, do not cause metabolic DDI when co-administered with otherdrugs;

[0041] 10. Compounds of the invention, as well as metabolites thereof,do not elevate LFT values when administered alone.

[0042] In some embodiments, the subject invention provides compoundsthat have any two of the above-identified characteristics or properties.Other embodiments provide for compounds having at least any three of theabove-identified properties or characteristics. In another embodiment,the compounds, and compositions thereof, have any combination of atleast four of the above-identified characteristics or properties.Another embodiment provides compounds having any combination of five toten of the above-identified characteristics or properties. In apreferred embodiment, the compounds of the invention have all tencharacteristics or properties.

[0043] In various embodiments, the primary metabolites of the inventivecompounds, regardless of the electrophysiological properties of theparent drug, has, or have, negligible inhibitory activity at the IK_(R)(HERG) channel at normal therapeutic concentrations of the drug inplasma. Preferably, the concentration of the metabolite must be at leastfive times higher than the normal therapeutic concentration of theparent compound before activity at the IK_(R) channel is observed.Preferably, the concentration of the metabolite must be at least tentimes higher than the normal therapeutic concentration of the parentcompound before activity at the IK_(R) channel is observed.

[0044] Compounds according to the invention are, primarily, metabolizedby endogenous hydrolytic enzymes via hydrolysable bonds engineered intotheir structures. The primary metabolites resulting from this metabolicpathway are water soluble and do not have, or show a reduced incidenceof, DDI when administered with other medications (drugs). Non-limitingexamples of hydrolysable bonds that can be incorporated into compoundsaccording to the invention include amide, ester, carbonate, phosphate,sulfate, urea, urethane, glycoside, and other bonds that can be cleavedby hydrolases.

[0045] Additional modifications of the compounds disclosed herein canreadily be made by those skilled in the art. Thus, analogs and salts ofthe exemplified compounds are within the scope of the subject invention.With a knowledge of the compounds of the subject invention skilledchemists can use known procedures to synthesize these compounds fromavailable substrates. The accompanying figures show certain specificcompounds including those substituted with lower (C₁₋₄) alkyl. Theperson skilled in the art having the benefit of the instant disclosurewould appreciate that other substituents could be made in order toarrive at other compounds having the advantageous biological activity(5-HT3 receptor antagonist) and pharmacokinotic properties.

[0046] As used in this application, the term “analogs” refers tocompounds which are substantially the same as another compound but whichmay have been modified by, for example, adding additional side groups.The term “analogs” as used in this application also may refer tocompounds which are substantially the same as another compound but whichhave atomic or molecular substitutions at certain locations in thecompound.

[0047] Analogs of the exemplified compounds can be readily preparedusing commonly known, standard reactions. These standard reactionsinclude, but are not limited to, hydrogenation, methylation,acetylation, and acidification reactions. For example, new salts withinthe scope of the invention can be made by adding mineral acids, e.g.,HCl H₂SO₄, etc., or strong organic acids, e.g., formic, oxalic, etc., inappropriate amounts to form the acid addition salt of the parentcompound or its derivative. Also, synthesis type reactions may be usedpursuant to known procedures to add or modify various groups in theexemplified compounds to produce other compounds within the scope of theinvention.

[0048] The subject invention further pertains to enantiomericallyisolated compounds, and compositions comprising the compounds, for 5-HT3antagonism. The isolated enantiomeric forms of the compounds of theinvention are substantially free from one another (i.e., in enantiomericexcess). In other words, the “R” forms of the compounds aresubstantially free from the “S” forms of the compounds and are, thus, inenantiomeric excess of the “S” forms. Conversely, “S” forms of thecompounds are substantially free of “R” forms of the compounds and are,thus, in enantiomeric excess of the “R” forms. In one embodiment of theinvention, the isolated enantiomeric compounds are at least about in 80%enantiomeric excess. In a preferred embodiment, the compounds are in atleast about 90% enantiomeric excess. In a more preferred embodiment, thecompounds are in at least about 95% enantiomeric excess. In an even morepreferred embodiment, the compounds are in at least about 97.5%enantiomeric excess. In a most preferred embodiment, the compounds arein at least 99% enantiomeric excess.

[0049] A further aspect of the subject invention pertains to thebreakdown products which are produced when the therapeutic compounds ofthe subject invention are acted upon by hydrolytic enzymes, such asesterases. The presence of these breakdown products in urine or serumcan be used to monitor the rate of clearance of the therapeutic compoundfrom a patient.

[0050] The compounds of this invention have therapeutic propertiessimilar to those of the unmodified parent compounds. Accordingly, dosagerates and routes of administration of the disclosed compounds aresimilar to those already used in the art and known to the skilledartisan (see, for example, Physicians' Desk Reference. 54^(th) Ed.,Medical Economics Company, Montvale, N.J., 2000).

[0051] The compounds of the subject invention can be formulatedaccording to known methods for preparing pharmaceutically usefulcomposition. Formulations are described in detail in a number ofsources, which are well known and readily available to those skilled inthe art. For example, Remington's Pharmaceutical Science by E. W. Martindescribes formulation, which can be used in connection with the subjectinvention. In general, the compositions of the subject invention areformulated such that an effective amount of the bioactive compound(s)composition.

[0052] In accordance with the subject invention, pharmaceuticalcompositions are provided which comprise, as an active ingredient, aneffective amount of one or more of the compounds and one or morenon-toxic, pharmaceutically acceptable carriers or diluents. Examples ofsuch carriers for use in the invention include ethanol, dimethylsulfoxide, glycerol, silica, alumina, starch, and equivalent carriersand diluents. Further, acceptable carriers can be either solid orliquid. Solid form preparations include powders, tablets, pills,capsules, cachets, suppositories and dispersible granules. A solidcarrier can be one or more substances, which may act as diluents,flavoring agents, solubilizers, lubricants, suspending agents, binders,preservatives, tablet disintegrating agents or encapsulating materials.

[0053] The disclosed pharmaceutical compositions may be subdivided intounit doses containing appropriate quantities of the active component.The unit dosage form can be a packaged preparation, such as packetedtablets, capsules, and powders in paper or plastic containers or invials or ampoules. Also, the unit dosage can be a liquid basedpreparation or formulated to be incorporated into solid food products,chewing gum, or lozenges.

[0054] The subject invention further provides methods of synthesizingthe unique and advantageous therapeutic compounds of the subjectinvention. Particularly, methods of producing less toxic therapeuticagents comprising introducing ester groups into therapeutic agents aretaught. The ester linkage may be introduced into the compound at a sitewhich is convenient in the manufacturing process for the compounds ofthe invention. Various exemplary synthetic routes for the preparation ofthe compounds of the subject invention are described in FIGS. 6-8.Additionally, the sensitivity of the ester linkage may be manipulated bythe addition of side groups which hinder or promote the hydrolyticactivity of the hydrolases or esterases responsible for cleaving thedrug at the ester locus. Methods of adding such side groups, as well asthe side groups themselves, are well known to the skilled artisan andcan be readily carried out utilizing the guidance provided herein.

[0055] All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

[0056] It should be understood that the examples and embodimentsdescribed herein are for illustrative purposes only and that variousmodifications or changes in light thereof will be suggested to personsskilled in the art and are to be included within the spirit and purviewof this application.

We claim:
 1. A 5-HT3 receptor antagonist having at least onecharacteristic chosen from the group consisting of: a. the compound ismetabolized both by CYP450 and by a non-oxidative metabolic enzyme orsystem of enzymes; b. the compound has a short (up to four (4) hours)non-oxidative metabolic half-life; c. the compound contains ahydrolysable bond that can be cleaved non-oxidatively by hydrolyticenzymes; d. the primary metabolites of the compound result from thenon-oxidative metabolism of the compound; e. the primary metabolites aresoluble in water at physiological pH; f. the primary metabolites havenegligible inhibitory activity at the IK_(R) (HERG) channel at normaltherapeutic concentration of the parent drug in plasma; g. the compound,as well as the metabolites thereof, does not cause metabolic DDI whenco-administered with other drugs; and h. the compound, as well asmetabolites thereof, does not elevate LFT values when administeredalone.
 2. The compound, according to claim 1, wherein said compound hasthe following structure, or is an analog or a salt of a compound havingthe following structure:


3. The compound, according to claim 2, having the following formula:


4. The compound, according to claim 1, wherein said compound has thefollowing structure, or is an analog or a salt of a compound having thefollowing structure:


5. The compound, according to claim 4, having the following structure:


6. The compound, according to claim 1, wherein said compound has thefollowing structure, or is an analog or a salt of a compound having thefollowing structure:


7. The compound, according to claim 6, having the following structure:


8. A pharmaceutical composition comprising a 5-HT3 receptor antagonisthaving at least one characteristic chosen from the group consisting of:a. the compound is metabolized both by CYP450 and by a non-oxidativemetabolic enzyme or system of enzymes; b. the compound has a short (upto four (4) hours) non-oxidative metabolic half-life; c. the compoundcontains a hydrolysable bond that can be cleaved non-oxidatively byhydrolytic enzymes; d. the primary metabolites of the compound resultfrom the non-oxidative metabolism of the compound; e. the primarymetabolites are soluble in water at physiological pH; f. the primarymetabolites have negligible inhibitory activity at the IK_(R) (HERG)channel at normal therapeutic concentration of the parent drug inplasma; g. the compound, as well as the metabolites thereof, does notcause metabolic DDI when co-administered with other drugs; and h. thecompound, as well as metabolites thereof, does not elevate LFT valueswhen administered alone; wherein said composition further comprises apharmaceutical carrier.
 9. The pharmaceutical composition, according toclaim 8, comprising a compound, or an analog or salt thereof, whereinsaid compound has the following structure:


10. The composition, according to claim 10, comprising a compound havingthe following structure:


11. The composition, according to claim 8, comprising a compound or ananalog or salt thereof, wherein said compound has the followingstructure:


12. The composition, according to claim 11, comprising a compound havingthe following structure:


13. The composition, according to claim 1, comprising a compound, or ananalog or salt thereof, wherein said compound has having the followingstructure:


14. The composition, according to claim 13, comprising a compound havingthe following structure:


15. A method for blocking 5-HT3 activity in a patient in need of suchtreatment wherein said method comprises administering to said patient a5-HT3 blocking compound having at least one of the followingcharacteristics: a. the compound is metabolized both by CYP450 and by anon-oxidative metabolic enzyme or system of enzymes; b. the compound hasa short (up to four (4) hours) non-oxidative metabolic half-life; c. thecompound contains a hydrolysable bond that can be cleavednon-oxidatively by hydrolytic enzymes; d. the primary metabolites of thecompound result from the non-oxidative metabolism of the compound; e.the primary metabolites are soluble in water at physiological pH; f. theprimary metabolites have negligible inhibitory activity at the IK_(R)(HERG) channel at normal therapeutic concentration of the parent drug inplasma; g. the compound, as well as the metabolites thereof, does notcause metabolic DDI when co-administered with other drugs; and h. thecompound, as well as metabolites thereof, does not elevate LFT valueswhen administered alone.
 16. The method, according to claim 15,comprising administering a compound, or an analog or salt thereof,wherein said compound has the following structure:


17. The method, according to claim 16, wherein said compound has thefollowing formula:


18. The method, according to claim 15, comprising administering acompound, or an analog or salt thereof, wherein said compound has thefollowing structure:


19. The method, according to claim 18, wherein said compound has thefollowing structure:


20. The method, according to claim 15, comprising administering acompound, or an analog or salt thereof, wherein said compound has thefollowing structure:


21. The method, according to claim 20, wherein said compound has thefollowing structure:


22. The method, according to claim 15, wherein the patient is human. 23.The method, according to claim 15, wherein said method is used to treatirritable bowel syndrome.